This invention related to packaging of optoelectronic components which generate or process signals that pass through optical fibers. In particular, it addresses the critical need for providing stable, low-cost alignment of multiple single-mode optical fibers to a single packaged device, such as a semiconductor array of laser amplifiers, lasers or photodetectors. Such devices have closely spaced active regions to which the optical fibers must be coupled.
An optoelectronic package is a container or housing that provides protection and support for both active and passive components contained within it. These components and their interconnection represent an optical-electrical circuit and define the function of the package. The package also includes a means of connecting the internal components with the external environment, usually as electrical feed-through and optical fiber. Our invention is concerned with the optical fiber and how it is connected to the components within the package.
To make an optical connection between an optical fiber and an optoelectronic component within a package, it is necessary to position or align the optical fiber in a way that allows efficient coupling between the optical fiber and the optoelectronic component. The precision needed for the alignment depends on the size of the light-emitting or light receiving elements, the type of optical fiber, and any type of focusing or defocusing elements which may be present. Optical fiber transmits light through its inner core, which is much smaller than the diameter of the optical fiber. There are two classes of optical fiber presently used in packaging semiconductor devices: single-mode and multi-mode, with typical core diameters of about 10 microns and 50 microns, respectively. Many telecommunication applications use single-mode optical fiber because of the superior bandwidth arising from its reduction of mode partition noise.
The prior art for multi-fiber array alignment to a single package is predominantly concerned with the easier task of coupling large-core multi-mode optical fiber to relatively large light sources and detectors. These alignments are less sensitive to position and can often be done with grooved parts and epoxy to fasten the optical fiber. This technology is acceptable for short length optical fiber links in local area networks or computers, but not for telecommunications.
Connecting single-mode optical fiber to semiconductor devices is difficult. Extremely tight tolerances, on the order of one micron, are needed due to the small size (about one micron) of the active region and the small optical fiber core. Optical fibers are usually actively aligned to the semiconductor component. This means that for the semiconductor laser, the laser is electrically biased to emit light. The optical fiber is then aligned to a position that maximizes its reception of light, a condition monitored by coupling a photodetector to the opposite end of the optical fiber. The manipulation of the optical fiber is usually done with a suction-tipped micromanipulator arm with piezo-electric controls having submicron positional sensitivity. Additional problems arise when more than one optical fiber needs to be coupled to a single device, since this necessarily entails either simultaneous alignment or sequential alignment to multiple optical fibers. Simultaneous alignment is a situation in which each optical fiber must be physically connected to a manipulator of some kind, the optical fibers then moved together and then held in position all at the same time. Sequential alignment is the process of aligning separate optical fibers, one by one. Alignment of one optical fiber often disrupts previously aligned optical fibers such that the overall yield of the process may be low. For array alignments, the active elements may be only 150 to 300 microns apart on the semiconductor, leaving little room for holding the optical fibers, which normally have outside diameters (core plus cladding) of 125 microns. The optical fibers would be nearly in contact with each other when positioned for direct coupling to the active regions on the semiconductor.
Once single-mode optical fibers are aligned, they are usually fixed in their position by laser welding or soldering. It has been shown that the application of a graded index (GRIN) lens with an uptapered optical fiber will increase the alignment tolerances to the extent that the more easily made epoxy attachment can be made at room temperature and without the cost of laser welding. This advantage is present in our invention as applied to arrays.